Cancer is a leading cause of death in the developed world, with over one million people diagnosed and more than 500,000 deaths per year in the United States alone. Overall it is estimated that at least one in three people will develop some form of cancer during their lifetime. There are more than 200 different histopathological types of cancer, four of which (breast, lung, colorectal, and prostate) account for over half of all new cases in the U.S. (Jemal et al., Cancer J. Clin., 53,5-26 (2003)).
Many of these tumors arise from mutations that activate Ras proteins, which control critically important cellular signaling pathways that regulate growth and other processes associated with tumorigenesis. The name “Ras” is an abbreviation of “Rat sarcoma” reflecting the way the first members of the Ras protein family were discovered. The name “ras” also is used to refer to the family of genes encoding these proteins.
Ras-driven cancers have remained the most intractable diseases to any available treatment. New therapeutic and preventative strategies are urgently needed for such cancers (Stephen et al., Cancer Cell, 25, 272-281 (2014)). Drug discovery programs worldwide have sought Ras-selective drugs for many years, but heretofore no avail (Spiegel, et al., Nature Chem. Biol., 10, 613-622 (2014)). New drugs that selectively target abnormal or mutant Ras and/or Ras-mediated pathological processes in patients’ tumors will enable highly efficacious treatments of such patients while minimizing toxicity to cells and tissues with normal Ras functions (Stephen et al., supra; Spiegel et al., supra).
Ras proteins are key regulators of several aspects of normal cell growth and malignant transformation, including cellular proliferation, survival and invasiveness, tumor angiogenesis and metastasis (Downward, Nature Rev. Cancer, 3, 11-22 (2003)). Ras proteins are abnormally active in most human tumors due to mutations in the ras genes themselves, or in upstream or downstream Ras pathway components, or other alterations in Ras signaling. Targeted therapies that inhibit Ras-mediated pathways therefore are expected to inhibit the growth, proliferation, survival and spread of tumor cells having activated or mutant Ras. Some such new experimental therapeutic agents have shown promising activity in preclinical studies, albeit with only modest activity in human clinical trials.
Genetic mutations in ras genes were first identified in human cancer over 3 decades ago. Such mutations result in the activation of one or more of three major Ras protein isoforms, including H-Ras, N-Ras, or K-Ras, that turn on signaling pathways leading to uncontrolled cell growth and tumor development. Activating ras gene mutations occur de novo in approximately one third of all human cancers and are especially prevalent in pancreatic, colorectal, and lung tumors. Ras mutations also develop in tumors that become resistant to chemotherapy and/or radiation, as well as to targeted therapies, such as receptor tyrosine kinase inhibitors (Gysin et al., Genes Cancer, 2, 359-372 (2011)). While ras mutations are relatively infrequent in other tumor types, for example, breast cancer, Ras can be pathologically activated by certain growth factor receptors that signal through Ras.
Although ras gene mutations have been known for many years, there currently are no available cancer therapeutics approved by the U.S. Food and Drug Administration that are known to selectively suppress the growth of tumors driven by activated Ras. In fact, Ras has been described as “undruggable” because of the relative abundance in cells and high affinity for its substrate, GTP (Takashima and Faller, Expert Opin. Ther. Targets, 17, 507-531 (2013)).
In addition to its role in cancer, activated Ras is important in a variety of other diseases, collectively referred to as “rasopathies.” One such disease, neurofibromatosis type 1 (NF1), a very prevalent autosomal dominant heritable disease, is caused by a mutation in neurofibromin, a Ras GAP (inactivating protein), which results in Ras hyperactivation in the relatively common event of loss of the second NF1 allele. Such mutations reportedly affect 1:3000 live births. The most dire symptoms associated with NF1 include numerous benign tumors (neurofibromas) arising from precursor nerve cells and Schwann cells of the peripheral nervous system. These tumors can cause severe problems depending on their location within the body, such as hearing or vision loss, as well as disfiguring masses on visible areas. Less common but extremely serious complications may arise when central nervous system gliomas develop or plexiform neurofibromas become transformed, resulting in the development of metastatic peripheral nerve sheath tumors (Tidyman and Rauen, Curr. Opin. Genet. Dev., 19, 230-236 (2009)). Another rare developmental disease which is attributable to hyperactive H-Ras is Costello syndrome. This condition causes a range of developmental abnormalities as well as predisposing patients to a variety of benign and malignant neoplasms (Tidyman and Rauen, supra).
Several approaches to treat diseases that arise from activating ras mutations have been undertaken. Because full maturation of the Ras protein requires lipid modification, attempts have been made to target this enzymatic process with inhibitors of farnesyl transferase and geranylgeranyltransferase, but with limited success and significant toxicity. Targeting of downstream components of Ras signaling with inhibitors of Raf/Mek/Erk kinase components of the cascading pathway has been an extremely active area of pharmaceutical research, but also fraught with difficulties and paradoxes arising from complex feedback systems within the pathways (Takashima and Faller, supra).
Inhibitors targeting components within the PI3K/Akt pathway also have not been successful as single agents, but presumably might synergize with Raf/Mek/Erk pathway inhibitors to block Ras-dependent tumor growth and survival. Similarly, several other molecular targets have been identified from RNAi screening, which might provide new opportunities to inhibit the growth of Ras-driven tumors; such other potential targets include CDK4, Cyclin D1, Tiam1, Myc, STK33, and TBK, as well as several genes involved in mitosis (Takashima and Faller, supra).
The nonsteroidal anti-inflammatory drug, sulindac (FIG. 1) has been reported to selectively inhibit proliferation of cultured tumor cells having ras mutations (Herrmann et al., Oncogene, 17, 1769-1776 (1998)). Extensive chemical modifications of sulindac and the related NSAID, indomethacin, have been aimed at removing cyclooxygenase-inhibitory activity, while improving anticancer activity (Gurpinar et al., Mol. Cancer Ther., 12, 663-674 (2013); Romeiro et al., Eur. J. Med. Chem., 44,1959-1971 (2009); Chennamaneni et al., Eur. J. Med. Chem., 56, 17-29 (2012)). An example of a highly potent antiproliferative derivative is a hydroxy-substituted indene derivative of sulindac, OSIP-487703 (FIG. 1), that was reported to arrest colon cancer cells in mitosis by causing microtubule depolymerization (Xiao et al., Mol. Cancer Ther., 5, 60-67 (2006)). OSIP-487703 also was reported to inhibit the growth and induce apoptosis of human SW480 colon cancer cells. These properties of mitotic arrest and microtubule disruption were shared by several additional related compounds, including a pyridine (CP461) and trimethoxy (CP248) substituted variants (FIG. 1) (Lim et al., Clin. Cancer Res, 9, 4972-4982 (2003); Yoon, et al., Mol. Cancer Ther., 1, 393-404 (2002)). However, there was no reported association of antitumor properties of these compounds (FIG. 1) with Ras function, but rather such properties were attributed to direct binding to the microtubule subunit, tubulin, thereby causing mitotic arrest and blocking cell division. Still other reports describe their ability to induce apoptosis by inhibition of cGMP phosphodiesterase (Thompson et al., Cancer Research, 60, 3338-3342 (2000)).
Other investigators reported that sulindac sulfide (FIG. 1) can inhibit Ras-induced malignant transformation, possibly by decreasing the effects of activated Ras on its main effector, the c-Raf-lkinase, due to direct binding to the ras gene product p21 in a non-covalent manner (Herrmann et al., supra). Sulindac sulfide also can inhibit focus formation, a marker of malignant transformation, by rat or mouse fibroblasts by forced Ras expression, but not by other transformation pathways (Gala et al., Cancer Lett., 175, 89-94 (2002); Herrmann et al., supra). Sulindac sulfide was reported also to bind Ras directly and interfere with nucleotide exchange. Several groups additionally reported that sulindac interferes with Ras binding to the downstream signaling kinase c-Raf, and blocks activation of downstream signaling or transcription (Herrmann et al., supra; Pan et al., Cell Signal., 20, 1134-1141 (2008)).
The aforementioned findings led to efforts to improve the Ras inhibitory activity of sulindac sulfide through chemical modifications (Karaguni et al., Bioorg. Med. Chem. Lett., 12, 709-713 (2002)). Several derivatives were identified that were more potent inhibitors of tumor cell proliferation, and four related compounds (FIG. 2) exhibited selectivity towards a Ras-transfected MDCK cell line compared to the parental cell line. Three of these compounds also potently disrupted the Ras-Raf interaction. However, none of the four were more potent toward the mutant K-Ras-bearing SW-480 cell line, although they did inhibit Erk phosphorylation and bound weakly to the G-domain of H-Ras (Waldmann et al., Angew. Chem. Int. Ed. Engl., 43, 454-458 (2004)).
In addition to sulindac sulfide, the non-COX inhibitory sulfone metabolite of sulindac has been reported to have selective effects on tumor cells with mutant Ras. For example, transfection of Caco-2 colon tumor cells with the activated K-Ras oncogene caused cells treated with either sulindac sulfide or sulfone to undergo apoptosis earlier than non-transfected cells. (Lawson et al., Cancer Epidemiol. Biomarkers Prey., 9, 1155-62 (2000)). Other investigators have reported that sulindac sulfone can inhibit mammary tumorigenesis in rats and that the effect was greater on tumors with the mutant H-Ras genotype (Thompson et al., Cancer Research 57, 267-271 (1997)). However, other investigators report that the inhibition of colon tumorigenesis in rats by either sulindac or sulindac sulfone occurs independently of K-Ras mutations (de Jong et al., Amer. J. Physio. Gastro and Liver Phys. 278, 266-272 (2000)). Yet other investigators report that the K-Ras oncogene increases resistance to sulindac-induced apoptosis in rat enterocytes (Arber et al., Gastroenterology, 113, 1892-1900 (1997)). As such, the influence of Ras mutations on the anticancer activity of sulindac and its metabolites is controversial and unresolved, and has not been exploited to improve anticancer potency or selectivity.
Certain other compounds have been described with selective toxicity toward cells expressing activated Ras. A high-throughput phenotypic screen of over 300,000 compounds was conducted within NIH Molecular Libraries Screening Center program to identify compounds which were synthetically lethal to cells expressing oncogenic H-Ras. A lead compound, ML210 (FIG. 3), inhibited growth of cells expressing mutant Ras with an IC50 of 71 nM, and was 4-fold selective versus cells lacking oncogenic Ras. Though the specific molecular target of ML210 is unknown, the compound was chemically optimized to eliminate reactive groups and improve pharmacologic properties (ML210, 12/12/2011 update, Probe Reports from NIH Molecular Libraries Program, Bethesda, http://www.ncbi.nlm.nih.gov/books/NBK98919/).
A separate high-throughput screen identified two compounds, RSL3 and RSLS (FIG. 3) which induce non-apoptotic, Mek-dependent, oxidative cell death (Yang and Stockwell, Chem. Biol., 15, 234-245 (2008). RSLS, like a previously identified Ras synthetic lethal compound, erastin (FIG. 3), binds the voltage-dependent anion channel (VDAC) (Dolma et al., Cancer Cell, 3, 285-296 (2003)). Yet another small-molecule screen identified oncrasin, a compound selectively active against K-Ras mutant cell lines (Guo et al., Cancer Res., 68, 7403-7408 (2008)). One analog, NSC-743380 (FIG. 3), is highly potent and has shown anti-tumor activity in a preclinical model of K-Ras driven renal cancer (Guo et al., PLoS One, 6, e28487 (2011)). A prodrug approach has recently been described for oncrasin derivatives, to improve stability, pharmacokinetics, and safety (Wu et al., Bioorg. Med. Chem., 22, 5234-5240 (2014)). A synthetic lethal screen using embryonic fibroblasts derived from mice expressing the oncogenic K-Ras (G12D) identified a compound, lanperisone (FIG. 3), that induced non-apoptotic cell death via a mechanism involving oxidative stress (Shaw et al., Proc. Natl. Acad. Sci. USA, 108, 8773-8778 (2011)). In contrast to the synthetic lethal approach, a fragment-based screening approach paired with crystallographic studies has been used to identify compounds which irreversibly bind to and inhibit K-Ras in lung tumor cells having the relatively rare G12C ras gene mutation (Ostrem et al., Nature, 503,548-551 (2013)). While compounds of this series potently inhibit Ras through a covalent interaction, the low frequency of this mutation may limit the utility of such compounds. Finally, a new investigational strategy for targeting oncogenic Ras has been described (Zimmerman et al., J. Med. Chem., 57, 5435-5448 (2014)) which involves structure guided design and kinetic analysis of benzimidazole inhibitors targeting the PDEδ prenyl binding site.
WO 97/47303 and WO 2014/047592, U.S. Patent Application Publication Nos. 2003/0009033 and 2003/0194750, U.S. Pat. Nos. 6,063,818; 6,071,934, 5,965,619; 5,401,774; 6,538,029, 6,121,321, and UK Patent No. GB 1370028 disclose certain anticancer compounds; however, these documents do not disclose that the compounds have any Ras-specific activity, nor any basis for a selective Ras-directed method of use.
The foregoing shows that there exists an unmet need for compounds that are suitable for treating or preventing cancers. There further exists an unmet need for compounds that inhibit Ras-dependent diseases or undesirable conditions.